Electron transport via quantum tunneling in metal insulator diodes

By Debra Vogler, Senior Technical Editor, Solid State Technology

The humble diode is the focal point of what Phiar Corp. hopes will be the beginning of a future that includes non-semiconductor materials for junction transport via quantum tunneling — and the first credible alternative to semiconductors since the vacuum tube era.

Potential applications of the company’s metal-double-insulator-metal (MIIM) diodes include:

- 60GHz+ wireless communications (addresses antenna-edge frequency conversion);
- millimeter wave radar;
- millimeter wave and Terahertz imaging and spectroscopy;
- next-generation Flash memory (enabling direct addressing at NAND densities); and
- chip-to-chip RF communications (reducing copper interconnects).

Motorola Labs has already gone on record validating the high-speed performance of MIIMs for future wireless applications. Vida Ilderem, VP of Motorola’s embedded systems research, said in a joint news release that Phiar’s diodes surpass the benchmarks of commercially available diodes for millimeter wave detector applications (see Fig. 1, above). A joint paper presented at the 2007 IEEE RFIC Conference discussing measurements taken at Motorola support that claim, noted Adam Rentschler, director of business development at Phiar.

MIIM devices are made of amorphous films deposited at temperatures below 300°C. Phiar says it has proven the technology on several substrates — CMOS, post-CMP CMOS, SiO2, quartz, and polyimde. RF signal integrity is optimized by locating passives, antennas, and detector diodes on the same substrate.

Devices built with the MIIM structure, which uses nanoscale stacks of metals and insulators instead of traditional compound semiconductor materials, offer two primary speed advantages over semiconductors, according to Rentschler. “First, traditional semiconductor junction transport is replaced by quantum tunneling, a phenomenon so fast it requires only about one femtosecond,” he told WaferNEWS. “Second, in metal-insulator devices, bulk electron transport takes place in metals, rather than much lower mobility semiconductor materials” (see Fig. 2, below).

Much of the projected major cost-savings in the MIIMs process flow come from the ability to use existing sputtering equipment as well as eliminating the need for exotic compound semiconductor materials, explained Rentschler. Additionally, because the nominal device linewidths are 300nm, there is no need for cutting-edge lithography. “Currently, we only need four mask layers — that number will probably start to increase a bit when we start building more sophisticated analog ICs, but that number is still small compared to the number of mask layers one would normally find in a typical CMOS process,” he said. The 60GHz diodes developed for Motorola measure roughly 300nm x 300nm, dimensions easily achieved with older manufacturing technology.

The company is careful, though, not to make claims about supplanting Moore’s Law. “Those working at the cutting edge of semiconductor technology continue to do amazing things and keep inventing around seemingly insurmountable challenges,” Rentschler told WaferNEWS. “Moore’s Law is alive and well, but there is now an alternative to incremental performance improvement in semiconductor technology. Metal-insulator electronics is a brand new platform for engineers to utilize, particularly in applications requiring low cost, high performance analog devices.”

RF engineers may become enthusiastic as this technology is deployed. According to the company, its metal-insulator electronics offers the ability to tap not only the 7GHz of unlicensed spectrum centered around 60GHz, but also the same-sized chunks the FCC has allocated at 120GHz and 244GHz. “The devices stay the same,” said Rentschler, “but the antennas shrink to handle the higher frequencies.” He added that Phiar plans to eventually build chip-to-chip RF interconnects at carrier frequencies of 500GHz or higher.

Another application being explored is flash memory, where “there’s always a need to achieve greater density and greater speeds,” says Rentschler. He wouldn’t elaborate on details, citing a confidentiality agreement, but noted that Phiar’s technology enables the fabrication of a very high-current density, very small-switching diode that’s compatible with CMOS and proprietary flash memory technology. – D.V.

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